In general, capacitors comprise two electrodes and a dielectric material between the electrodes. The dielectric material may be an insulator, ceramic or a gas. Capacitors may be used in MEMS devices such as MEMS microphones (MEMS=micro electro mechanical system), MEMS sensors or MEMS switches.
The capacitance of a capacitor having flat electrodes depends on the area of the electrodes, the dielectric material and the distance in between. It is often preferred that one or both electrodes are flat, such, that the distance between the electrodes is well controlled. That is, the electrodes should be flat and arranged in parallel.
MEMS capacitors can be assembled by complementary metal-oxide-semiconductor (CMOS) processing methods. WO 2010/092399 refers to a MEMS microphone comprising MEMS capacitors and methods for production of MEMS capacitors that are compatible with standard CMOS processing techniques. U.S. Pat. No. 7,851,247 B2 refers to methods of fabricating MEMS microphone structures comprising a metal interconnect layer on a dielectric layer.
However, the application of CMOS production steps often results in an uncontrollable mechanical stress in the assembled electrodes of a MEMS capacitor.
MEMS microphones usually comprise a stiff back plate as a first electrode and a compliant membrane as a second electrode. The compliant membrane may oscillate due to differences in air pressure of an acoustical signal. As a result of this oscillation the distance between the electrodes varies correspondingly. Thus, an acoustical signal can be converted into an electrical signal by evaluating the varying capacity of the MEMS capacitor.
The problem of conventional CMOS compatible produced back plates is a mechanical stress in the assembled back plate. Using CMOS compatible material the mechanical stress is, in general, compressive and thus the back plate structure bends or buckles in an uncontrollable manner. Then, the distance between membrane and back plate becomes uncontrollable. As a result of this, the performance of the microphone is degraded.